Many products require flat panel displays to display video, computer or other data. Liquid crystal displays, LCDs, have become the dominant technology utilized in flat panel displays. Another, less common technology used for flat panel displays is plasma technology. Other known display technologies used in thicker flat panel displays are of the rear projection type. For very large displays, discrete arrays of LEDs are the dominant technology. These display technologies are used in many types of products including cellular phones, laptop computers, computer monitors, TVs, large commercial displays, and billboards. CRT type displays that used to be the dominant technology have almost vanished, even though the performance of the newer technologies is not significantly greater than CRT. Some current art LCD displays still cannot match the refresh rate of CRTs.
Displays based on LCD technology have been evolving for decades. Thousands of patents have been filed on improvements to the basic technology. Still, the performance of these displays lacks in many ways.
A first shortcoming of LCD display technology is the high energy consumption. A 65″ diagonal HDTV LCD TV typically draws around a half of a kilowatt. This is a result of the poor efficiency of the technology. LCDs need polarized light to function. However, approximately half of the light generated by the backlight is absorbed in the creation of polarized light. Many inventions have been devised to reduce this loss. In reality, little real improvement has been realized by manufactures due to cost or the reduction in other performance parameters. One product that is designed to recycle light not having the correct polarization is called ADBEF″ and is manufactured by 3M of Minneapolis, Minn.
Another factor that contributes to the low efficiency of LCD displays is the fact that pixels that are turned off absorb light rather than reflect it to another pixel that is on.
Another shortcoming of LCD displays is their limitations when used with color filters. Typically red, green, and blue filters are used to create colors. These filters do not reflect unused light, but rather absorb it. For example: red filters absorb the green and blue light while only letting red light pass through. In theory, a perfect blue filter would let 33% of the light through. In reality filter materials perform significantly less than the theoretical 33%. Another place where light is absorbed is in the matrix between the color filters. This matrix area is required for circuitry and transistors used to control the pixels. The required area is significant in that one pixel requires three transistors, one transistor being required for each of the three colors. Also, additional circuitry is required to drive the transistors. The matrix area between the filters may absorb approximately half the overall light available. When all of these and other losses such as reflection and material absorption are taken into consideration, an LCD panel may only be 8% efficient when all of the pixels are on. Typically an image has approximately one half of the pixels turned on when creating an image, and with the half of the pixels that are off absorbing rather than reflecting, the resulting LCD efficiency is only in the 4% range.
This poor efficiency requires the backlights used with LCD displays to be large and powerful. The dominant lamp technology used in displays is fluorescent type lamps. These lamps are reasonably efficient but require mercury. The mercury creates a disposal problem. In many cases, the mercury ends up in our food chain.
Another deficiency with LCD technology is the refresh rate. Only in the very recent past have LCDs been able to equal the refresh rate of CRT displays. For demanding applications such as the viewing of moving videos, the slow refresh rate of LCDs is apparent. Other problems with LCDs are poor contrast ratios. The contrast problem is exacerbated when viewed from a position off normal to the display surface.
The quality of the colors from an LCD display is limited by the wavelengths of light emitted from the light source and the properties of the color filters used in the display. Both of these factors result in displays that cannot accurately reproduce colors found in nature.
Another deficiency with LCD technology is its limited environmental operating range. The liquid crystal material doesn't function well at high and low temperatures. Displays that are used in extreme environments are often cooled or heated to keep them within a moderate operating range. Another problem with using LCDs in non-optimal environments is that the polarizing films required for LCD displays degrade when exposed to high humidity. Measures must be taken to reduce the effect of this property. In displays that are used in extreme environments the displays and their polarizing films are encased in glass windows.
Plasma thin panel display technology is the typical technology of choice for large screen TVs. The plasma displays also consume a significant amount of power. Plasma TVs do not last as long as LCD TVs and experience “burn in”. Burn in occurs when the on pixels remain on for a long period of time. These pixels lose their strength and become washed out over time. Cost is another issue with plasma technology.
In TV applications the projector is often deployed in a rear projection configuration. For computer monitors using projection display, the front projection mode is more commonly used.
Most rear and front projection displays utilize a MEMS mirror array. A MEMS mirror array is disclosed in each of U.S. Pat. Nos. 4,566,935; 4,596,992; 4,615,595; 4,662,746; 4,710,732; 4,956,619; and 5,028,939; all by inventor Larry Hornbeck of Texas, and assigned to Texas Instruments (TI) of Texas. The TI technology uses an array of MEMS mirrors that change their incidence angle to the light path to switch the light from an off position to an on position. When the mirror is in the on position, the mirror reflects the light through the optical path. When the mirror is in the off position, the light is reflected to a path that falls outside the projection optics. This in effect turns the light valve to an off state.
There are many deficiencies with this technology. One is that the light transmission is less than 70%. To allow for the change of angular orientation of the mirrors, there must be a substantial space between adjacent mirrors. The required gap causes a lot of light to be wasted. Further, the reflected light is absorbed into the light valve. The absorbed energy makes cooling the switching devices that use this technology a challenge.
Another flat panel display technology is disclosed in U.S. Pat. No. 5,319,491 by inventor Martin Selbrede of Thousand Oaks, Calif. This patent discloses a method in which the shape of an elastic membrane is changed to allow light to escape from a light guide. It is difficult to control the shape of the elastomer and therefore difficult to control the light output from the pixel. Light output from the pixels is dependent on the angle at which the light strikes the membrane. Also the angle at which the light exits the panel is off from normal. Typically light normal to the screen is the orientation in which you want the most output. Contrast ratio is limited with the elastic membrane technology. This limitation is due to the fact that any flaw in the light or optics lets light escape. An extremely small defect can produce enough light leakage to result in poor contrast when the display is primarily black. In high ambient lighting conditions the contrast is reduced by another factor. This factor is that the deformed elastomer will reflect, in some instances, the ambient light to the viewer.
Another flat panel display technology is disclosed in U.S. Pat. Nos. 6,040,937; 6,674,562; 6,867,896; and 7,124,216 all by inventor Mark Miles of Boston, Mass. This invention controls the distance between optical elements to control the interference characteristics of the pixel. This technology is only effective in a reflective mode and is therefore not applicable to most display applications. Three optical switches are required to create red, green, and blue colors. Not only are three-color optical switches required, but also the electronics to drive the switches must also be included.
Another display invention was recently disclosed in U.S. Publications 20050248827, and 20060070379, both by inventor Gary Starkweather of WA and assigned to Microsoft, also of WA. This technology is similar to the Hornbrook technology in that it switches light by bending or moving mirrors. This technology suffers due to its high complexity and therefore high cost. The advantage of this technology is that its theoretical efficiency is better than most other technologies. But in practice, the technology requires a collimated backlight source. Sources of this type are inefficient and costly. The cost of a display with this technology will be high and the efficiency still poor. Further, the creation of a collimated backlight source requires that there be considerable depth to the display. This depth is not desirable to consumers and therefore reduces the market for this technology.
The current invention utilizes micro-optical components. Some of the prior art related to this field should also be discussed. U.S. Pat. No. 6,421,103, by Akira Yamaguchi of Japan and assigned to Fuji Film, discloses a backlight for use with LCD panels. This patent discloses light sources, a substrate, apertures (not used as a light guide), and reflective regions on the substrate. The light is either reflected by the reflective surface or passes through the apertures. The light that passes through the apertures is captured by a lens and is used to control the direction of the light. The Yamaguchi reference teaches a restricted angle of the light to concentrate more light directly at the viewer of an LCD type display.
U.S. Pat. No. 5,396,350, by Karl Beeson of Princeton, N.J., discloses a light guide with optical elements that are used to extract light from the light guide. The optical elements are on the viewer's side of the panel and have limited ability to control the direction of the light. This invention is intended to be used in conjunction with an LCD type panel to concentrate light towards the viewer.